Conventionally various techniques have been proposed for retrieving similar images. For instance, in the techniques that are put in practical use to some extent for retrieving similar images with respect to photograph images, most techniques use color data as an image feature amount. Further, most techniques take a histogram regarding color data and perform image retrieval based on an RGB ratio as well as a combination of colors that exist a lot in the image. However, in such techniques, since color position data is lost, the retrieval accuracy is not necessarily high.
A similar image retrieving method that takes a layout of image feature amounts into consideration has been disclosed (Japanese Patent Application Laid-Open No. 11-288418). This method has achieved some improvement on retrieving an image having a difference in color layout.
Meanwhile, the following system has been conceived for comparing original image data with a paper image where image data has been printed on paper (hereinafter referred to as a paper image).
Conventionally, when a document is to be generated using a paper image, the paper image is scanned and converted to image data, and then the image data is pasted on a document. Moreover, when a printed paper image is to be copied, the paper image is copied (i.e., scanned and printed) by a copying machine. However, in general, printing image data causes image quality deterioration and color changes when compared to original image data. Similarly, scanning a paper image and converting it to image data also causes image quality deterioration and color changes when compared to the paper image.
These phenomena take place because the color gamut used by the original image data, the color gamut that can be outputted by a printer, and the color gamut that can be scanned by a scanner do not completely match. In order to suppress such image quality deterioration and color changes as much as possible and to generate an excellent-quality document or copy, it is necessary to reduce the system of printing and scanning as much as possible. More specifically, when image data is to be pasted on a document, original image data has to be pasted; and when an image is to be copied, original image data has to be printed. For this purpose, an original image data retrieving technique is necessary.
For another example, a system applied to data management has been conceived. In this system, printed image data, scanned image data, copied image data and the like are registered in a database along with information such as date and time, a user name and the like. Then, the date and time of printing, scanning or copying, and a user name of certain image data are inputted for a search. In such system, a technique is necessary to retrieve image data, obtained by scanning a paper image, based on an original image.
However, since the conventional image retrieving method strictly compares differences in colors, retrieval accuracy is not necessarily high in quality-deteriorated and color-changed images.